Lawn mowers have been known which cut grass, deliver the grass clippings, with carrier air produced by rotation of a cutter blade, from a grass clipping delivery passage into a grass bag and return only exhaust air from the grass bag to a cutter housing through an exhaust air return passage. In Japanese Utility Model Application Laid-open Publication No. SHO-61-125223, for example, there is disclosed a lawn mower which is constructed to return exhaust air from an upper rear portion of the grass bag to a central portion of the cutter housing in order to reduce unwanted sound and dust. The disclosed lawn mower will be described with reference to FIG. 21.
The lawn mower 300 shown in FIG. 21 is of an exhaust air circulation type, which includes a driving power source 302 disposed above the cutter housing (cutter deck) 301, a cutter blade 304 mounted on a shaft 303 of the driving power source 302, and a grass clipping delivery passage 305 provided in the cutter housing 301 for delivering grass clippings. Grass bag 307 for collecting therein grass clippings is attached to an outlet 306 of the grass clipping delivery passage 305, a return pipe 308 for returning exhaust air from the grass bag 307 to the cutter housing 301 extends from a rear upper portion of the grass bag 307, the distal end of the return pipe 308 is coupled to a middle portion of the cutter housing 301, and a fan 309 for drawing air from the return pipe 308 is fixed on the shaft 303 of the driving power source 302.
However, with the disclosed lawn mower 300, all of the carrier air can not be returned as exhaust air because the return pipe 308 for returning the exhaust air to the cutter housing 301 is very thin (and hence has a very small opening diameter or size). Thus, the lawn mower 300 can not achieve sufficient reduction in unwanted sound and dust by returning the carrier air, as exhaust air, from the grass bag 307 to the cutter housing 301. If the return pipe 308 for returning the exhaust air to the cutter housing 301 is made thicker (to have a greater opening diameter or size) in order to reduce unwanted sound and dust, then the overall weight of the lawn mower 300 would increase. Therefore, there is presently a demand for an improved lawn mower of the exhaust air return (circulation) type which can sufficiently reduce unwanted sound and dust and which can minimized an increase in weight inherent to the exhaust air circulation type.
Further, in U.S. Pat. No. 4,631,909, for example, there is disclosed another law mower which is constructed to reduce unwanted sound and dust by returning exhaust air from the grass bag to an upper portion of the cutter housing. The disclosed lawn mower will be discussed below with reference to FIG. 22.
The lawn mower 320 shown in FIG. 22 is of the exhaust air return (circulation) type, in which the cutter blade 322 is rotatably provided within the cutter housing 321 and the grass clipping delivery passage 323 for delivering grass clippings is provided in the cutter housing 321 along with an exhaust air return passage 324. Grass bag 327 is provided in communication with an outlet 325 of the grass clipping delivery passage 323 and inlet 326 of the exhaust air return passage 324. In this lawn mower 320, in order to separate the grass clippings and carrier air from each other, the grass bag 327 is formed of an air-permeable material, so that a portion of the carrier air is discharged out of the grass bag 327 while the remaining portion of the carrier air is returned to the cutter housing 321. Because of such a construction, the lawn mower 320 can not attain sufficient reduction in unwanted sound and dust. Therefore, it is necessary to reduce unwanted sound and dust more effectively for an improved working environment.
In some cases, the lawn mowing operation is performed with grass clippings left on a ground surface without being collected into the grass bag; such lawn mowing operation will be called “just-cutting lawn mowing operation”. However, in the operating state where the grass bag 327 is detached from the cutter housing 321 in the lawn mower 320 shown in FIG. 22, the exhaust air return passage 324 is left opened during the operation, which would disadvantageously make it difficult to secure necessary flows of the carrier air. Therefore, there is a demand for a lawn mower of enhanced convenience which can appropriately perform just-cutting lawn mowing operation.